The present invention relates to novel squarylium compounds which can be used in the optical recording field, and to optical recording media using the same.
In recent years, development of a digital versatile disc-recordable (DVD-R) as a recordable optical recording medium having a higher recording density than that of a compact disc-recordable (CD-R) has been under going. Both of CD-R and DVD-R are similar to each other in that an organic dye is utilized therein as a recording material and in a principle of recording and reproducing of a signal (information). Therefore, the organic dyes developed for CD-R can basically comply with the various requirements (light resistance, solubility, thermal decomposition properties) for the recording material of DVD-R other than spectroscopic properties. However, an oscillation wavelength of a semiconductor laser, which is used for recording the signal to DVD-R or for reproducing the signal from DVD-R, is in the range of 600-700 nm, which is shorter than that of the semiconductor laser which is used for CD-R. Accordingly, the recording material utilized for DVD-R should have an absorbance end of a longer wavelength side shorter than that of CD-R when it exists in the form of a membrane. Therefore, the dyes developed for CD-R such as cyanine dyes, azaannulene dyes and indoaniline-metal chelate dyes (xe2x80x9cElectronics Related Dyesxe2x80x9d, CMC, 1998) can not be used as the recording material for DVD-R.
The present inventors have developed squarylium compounds having different two kinds of aromatic substituents in a molecule. Such squarylium compounds have a squaric acid skeleton at a center of the molecule and substituents comprising an aromatic compound on carbon atoms at two catercornered positions of the skeleton. Squarylium compounds having two same aromatic substituents are conveniently referred to as symmetric squarylium compounds (or symmetric squarylium dyes), whereas those having different two kinds of substituents are referred to as asymmetric squarylium compounds (or asymmetric squarylium dyes).
The asymmetric squarylium compound having as a substituent an aromatic ring bound to a squaric acid skeleton with a carbon atom of the aromatic ring and an aromatic amine bound to the squaric acid skeleton with an amino group has been previously known (Japanese Unexamined Patent Publication No. 92914/1994). OXOCARBONS (Academic Press, 1980) describes a compound in which an aromatic substituent bound to the squaric acid skeleton with a carbon atom is phenyl. However, since the color of these compounds is yellow, brown or red, it is believed that the maximum absorption wavelength of these compounds is 500 nm or shorter and, therefore, these compounds can be scarcely used as a recording material for DVD-R from a viewpoint of the spectroscopic properties.
In addition, Japanese Unexamined Patent Publication No. 92914/1994 describes a compound in which an aromatic substituent bound to the squaric acid skeleton with a carbon atom is an azulenyl group. However, the molar extinction coefficient of such a compound is 4.8 or smaller and, therefore, the compound is unsuitable for the recording material for DVD-R from a viewpoint of the spectroscopic properties.
In view of an oscillation wavelength of the semiconductor laser used for DVD-R, for spectroscopic properties of the recording material, which have the close relation with recording and reproducing sensitivities of the signal, it is desirable that the maximum absorption wavelength (xcexmax) of the recording material measured in its solution is within the range of 550-600 nm and log xcex5 thereat (xcex5 is a molar extinction coefficient) is 5 or greater. In addition, for thermal decomposition properties of the recording material, which have the close relation with the recording sensitivity, it is desirable that a significant loss in weight is observed within the temperature range of 250-350xc2x0 C.
Furthermore, although light resistance and solubility in a solvent which is necessary for membrane formation are also required as the property of the recording material, there is no recording material having suitable properties for DVD-R, such as spectroscopic properties, light resistance, solubility and thermal decomposition properties, in the known squarylium compounds.
An object of the present invention is to provide squarylium compounds having spectroscopic properties, light resistance, solubility and thermal decomposition properties suitable as a recording material for DVD-R, and optical recording media using the same.
The present invention was done based on such a finding, and provides squarylium compounds represented by the formula (I): 
wherein, R1 and R2 are the same or different, and represent a hydrogen atom, an alkyl group optionally having a substituent, an aralkyl group optionally having a substituent, an aryl group optionally having a substituent or a heterocyclic group optionally having a substituent, or R1 and R2 may be taken together with an adjacent nitrogen atom to form a heterocycle, wherein the heterocycle may have a substituent, and R3 represents the general formula (II): 
wherein, R4 and R5 are the same or different, and represent a hydrogen atom or an alkyl group, or R4 and R5 may be taken together with an adjacent nitrogen atom to form a heterocycle, and R6, R7, R8 and R9 are the same or different, and represent a hydrogen atom, an alkyl group optionally having a substituent, an alkoxy group optionally having a substituent, a hydroxyl group or a halogen atom, or R4 and R6 or R5 and R7 may be taken together with adjacent Nxe2x80x94Cxe2x80x94C to form a heterocycle, wherein the heterocycle may have a substituent; or the general formula (III): 
wherein, R10 represents a carbon or nitrogen atom, R11 and R12 are the same or different, and represent a hydrogen atom, an alkyl group optionally having a substituent, an aryl group optionally having a substituent, an aralkyl group optionally having a substituent, or a hydroxyl group (provided that, when R10 is a nitrogen atom, then R12 is not present), R13 represents a hydrogen atom, an alkyl group optionally having a substituent, an aryl group optionally having a substituent or an aralkyl group optionally having a substituent, and R14 and R15 are the same or different, and represent a hydrogen atom, an alkyl group optionally having a substituent, an alkoxy group or a halogen atom, or R14 and R15 may be taken together with two adjacent carbon atoms to form an alicyclic hydrocarbon group, an aromatic ring optionally having a substituent or a heterocycle optionally having a substituent.
Also, the present invention provides optical recording media having a recording layer comprising said squarylium compound.